// 两个排序链表的归并
var mergeTwoList = function(l1,l2){
    if(l1 === null){
        return l2
    }
    if(l2 === null){
        return l1
    }
    if(l1.val < l2.val){
        l1.next = mergeTwoList(l1.next,l2)
        return l1
    }else {
        l2.next = mergeTwoList(l1,l2.next)
        return l2
    }
}
